Patent Literature 1 describes an oligonucleotide analog having stable and excellent antisense or antigene activity or having excellent activity as a detection reagent (probe) for a specific gene or as a primer for initiation of amplification of a specific gene, and the following nucleoside analog:
which serves as an intermediate for production of the oligonucleotide analog. The patent literature also describes steps A-3 and A-4 as a method for producing the nucleoside analog.

Patent Literature 1, however, makes no mention about a silyl protective group, though it specifically describes only an example wherein R7 is a p-toluenesulfonyloxy group. Particularly, the production method described in Patent Literature 1 disadvantageously has a significantly reduced reaction yield, when the nucleobase is a 2-isobutyrylamino-6-hydroxypurin-9-yl group (yield of 6% in two steps of Reference Example 15 and Example 24). In addition, it is less than industrially satisfactory, even when the nucleobase is any of other groups, for example, when the nucleobase is a 6-benzoylaminopurin-9-yl group, the yield of step A-3 is as low as 52%.
Patent Literature 2 describes a nucleoside analog with the 2′- and 4′-positions bridged by a NHCOCH2 group, and discloses the step of synthesizing compound 40 from compound 39 in Example 5(6).
